1. Field of the Invention
This invention pertains to vertical takeoff and landing (VTOL) airplane. More specifically, it pertains to a type of tilt-rotor VTOL airplane wherein large diameter helicopter-type of rotor is used for vertical lift, and which may be tilted 90 degrees forward during cruising flight when the aircraft is supported by conventional wings. In this invention, there is at least one large tiltable rotor attached to the aircraft provide vertical lift when the rotor is pointed upward and a separate means for providing horizontal thrust during cruise when the tiltable rotor is pointed 90 degrees horizontal, during which phase, the tiltable rotor is slowed down and is allowed to rotate at a minimum rotational rate adequate for maintaining structural integrity of the rotor blades.
2. Discussion of the Prior Art
Shortly after the airplane was invented, its disadvantage of requirement of a significant runway for takeoff and landing was quickly noticed, which significantly limit the airplane's utility. The helicopter was introduced afterward in order to overcome the limitation of the airplane. However, the helicopter has not received wide spread use but only in special roles that strictly require VTOL capability, and the helicopter numbers is but 1/10 that of the airplane. The helicopter flies too slowly and too inefficiently, with speed and range ½ to ⅓ that of the airplane, with 2 to 3 times the fuel consumption and cost of operation per passenger-mile. The helicopter is less safe per passenger-mile basis. According to NTSB statistics, the fatality rates for piston helicopters is 3–4/100,000 hrs and for light turbine helicopters 2–3/100,000 hrs where as the rates for a typical high wing airplane such Cessna 172 is 0.5 and Cessna 182 is 0.7/100,000 hrs. Light turbine helicopters have purchasing cost 2–4 times that of comparable piston airplane, but recently, the Robinson piston helicopters with their simplified rotor head design has brought down their purchasing cost to a level comparable with piston airplane.
In order to maintain the VTOL advantage of the helicopter while overcoming the helicopter's inefficiency and slow speed, there have been at least 50 different projects experimenting with high-speed VTOL aircraft by a large numbers of well known aerospace companies, proposing at least 12 different configurations in the last five decades. See, for example, “An Introduction to V/STOL Airplanes” by Iowa State University Press, 1981, or search the internet at http://www.vstol.org/. Today, there are only two VTOL transport airplanes that have sufficient merits to achieve production status, the military tilt-rotor Bell-Boeing V-22, and the civilian tilt-rotor Bell-Agusta BA-609 pending certification. On my last patent disclosure, U.S. Pat. No. 6,382,556, a single tilt-rotor VTOL design was disclosed which can improve the state of the art by potentially making VTOL airplane less expensive, more reliable and having more load capacity. However, my previous design relied on using the very large main rotor also as horizontal propulsion means in similar fashion as the V-22, which is less efficient and resulting in high level of adverse torque to the fuselage which must be overcome by the wing requiring more pilot's attention and making the wing less efficient. To be used as horizontal propulsion means, the main or lift rotor must have a highly twisted rotor blade downwardly from root to tip, and that will make the lift rotor somewhat less efficient in the VTOL mode, but far more importantly, the highly-twisted rotor blades cannot provide adequate autorotation performance in the same manner as would a conventional helicopter rotor in the event of engine failure. V-22 pilots are taught that in the unlikely event of failure of both engines, the aircraft should glide forward with the rotors in the horizontal orientation instead of attempting to autorotate with the rotors in the vertical orientation due to very high sink rate in the autorotation mode. If a suitably smooth landing site not found, a fatal crash landing will be likely due to the very high glide speed of the V-22 with very high wing loading and low wing aspect ratio. Thus, two engines must be provided for fail-safe purpose, which would be too expensive in a smaller size aircraft, or a provision must be made for variable twist in the rotor blade, which is difficult to do. A third reason for providing a separate horizontal propulsion means in a tilt-rotor aircraft would be to use a turbofan in order to increase the cruise speed of the tilt-rotor VTOL aircraft to a level comparable with current conventional turbofan jet aircraft. Propeller aircraft is limited to an efficient airspeed of around 350–400 mph, while a turbofan aircraft can cruise at 500 mph, due to the increase drag of the propeller tip when operating above the speed of sound. In the current tilt-rotor aircraft designs, the very large size of the rotor when used as forward propulsion device produces too much propulsion drag in the cruise mode, thereby limiting efficient cruise speed of current designs to around 300 mph.
Furthermore, the rotor tilting mechanism for a single-tilt-rotor airplane in this disclosure is an improvement from my previous design in that the it does not intrude into the center of the passenger cabin as did the previous design, while permitting nearly any type of engines to be used, ranging from the very bulky certified aircraft piston engine to twin turbofan engines. My previous single tilt-rotor design cannot utilize bulky piston engines and would be quite aesthetically-challenged even if compact twin turboshaft engines are used.
Thus the VTOL tilt-rotor airplane, the most successful VTOL airplane configuration, can be further improved as will be detailed within this disclosure in order to be simpler, cheaper, safer and having higher performance than current twin wing-mounted tilt-rotor design. It is hope that these major improvements in VTOL airplane design can make a significant impact on personal, business and commercial air transportation, thereby reducing the problem of airport congestion at large commercial airport and increasing rate of closure of small airports due to local political pressure favoring real-estate developers.
To my knowledge, there has not been any disclosure of VTOL single-tilt-rotor airplane having a separate means for horizontal propulsion in the cruising mode, while the main rotor is allowed to spin slowly at a minimum rate necessary for maintaining integrity of the rotor blades.
There have been unbuilt designs of twin wing-mounted tilt-rotor VTOL airplanes in the 1960's having the rotors stopped in flight after conversion of the rotors to the horizontal orientation, with the rotor blades subsequently folded into the engine nacelles for protection before accelerating to higher speed cruise, while turbofans are used for forward propulsion. Examples of such designs are the Sikorsky TRAC with telescoping rotor prior to rotor folding, the Bell T37 and D 272 designs with the rotor blades folded and trailed behind the engine nacelles, the Boeing USAF ARS (circa 1969), and numerous German designs of the same time frame. More details can be found at http://www.vstol.org/ in the section “Unbuilt V/STOL database.” All of these designs were eventually abandoned, perhaps due to the complexity and the strength requirement of an inflight rotor blade folding system. Today, we only see V-22 and the BA-609, utilizing a pair of tiltable prop-rotor for forward propulsion as well as for vertical lift. The long and slender rotor blades are too fragile to be left standing still and unfolded in high-speed cruise. To my knowledge, there has been no proposal of tilt-rotor aircraft having a separate horizontal propulsion means and having the tilt-rotor fully extended in cruising phase while allowed to rotate slowly at a minimum rotational rate necessary for the integrity of the rotor blade. Yet, this very simple arrangement requires no complexity built into the rotor blade, while allowing a slender, light-weight and flexible helicopter-style rotor blades to be used with all accompanying advantages, for example, the thin and flexible rotor blades put far less stress on the rotor hub and rotor nacelles and all mounting structures in comparison to more rigid rotor blades required for a stopped and folded rotor system as proposed. In a single engine VTOL airplane, this very simple arrangement is a must in order to allow autorotation in the event of cease of operation of the single engine.